i i i i a i Re ee eee 
_—————— ss rc‘é (<&P’””~=s— , 
ON THE ASCENT OF WATER IN TREES. 679 
fore preferable. But Askenasy seems to me to state the matter much 
more conveniently by using the term ‘imbibition.’! The force with which 
vegetable membranes, e.g., the thallus of Laminaria, absorb water has been 
demonstrated by Reinke and others, and the existence of such a force is 
familiar to botanists. 
Both Askenasy (loc. cit.) and Dixon and Joly? have pointed out that the 
force of imbibition, or the surface tension forces, as the case may be, can 
exert a tractional effect on the water in the tracheals, when the turges- 
cence of the mesophyll has been destroyed. But Askenasy in his original 
paper (1895), Dixon in the January 1896 paper, and again Askenasy in his 
second paper (March, 1896) have also considered the imbibitional or surface 
tension forces in connection with the turgescent cell. In his 1896 paper 
Dixon in fact gives up the view published in the Pil. rans. and adopts 
the view given by Askenasy in his original paper, that the tractional force 
is supplied by the osmotic suck of the leaves. It must clearly be under- 
stood that this does not remove imbibition from the problem. It is one 
of the chief merits of Askenasy’s work that he clearly sees and states the 
important relation between these forces. The sun’s heat causes the 
evaporation of the water with which the walls of the mesophyll cells are 
imbibed : this water is replaced by imbibition from the cell-sap. The con- 
centration of the cell-sap so produced maintains the osmotic torce of the 
cell, which again exerts suction on the water on the tracheals.! 
I have now given, in its simplest form, the modern theory of the rise 
of water. Apart from the main idea, it combines the points of several 
familiar views. Imbibition becomes a factor of paramount importance, 
though not in the way that Sachs employs it. The suspended threads 
of water remind us of Elfving’s capillary theory, while the living-element 
factor is represented by the turgescent mesophyll cells. 
Resistance.—It is not possible to discuss the question whether the 
tractional forces in the leaf are sufficient for the work imposed on them 
until we know what is the resistance to the passage of water through wood, 
For it is clear that the work done by the leaf includes, not only the lifting 
of a given column, but the overcoming of the resistance to its flow. 
The resistance to the flow of the transpiration current is in want of 
further investigation. Janse° has discussed the question, and points out 
(loc. cit., p. 36) that two kinds of resistance must be reckoned with. The 
first (which he calls statical) is illustrated by means of a cylinder of Pinus 
wood fixed to the short arm of a J tube filled with water, when it was 
found that in five days the level of water in the long arm was only 
1 mm. above that in the short arm.6 That is to say, when time enough 
is given, the resistance is practically nothing. Janse has also investigated 
the resistance to the passage of water flowing through wood at the rate of 
an ordinary transpiration current. His method seems to me open to criti- 
cism, but this is not the place to give my reasons. His experiments give 
a wide range of results. With Pinws strobus a pressure of water equal to 
ten times the length of the wood was required to force water through at 
1 Loe. cit. 1895, p. 10. 
? Annals of Bot. Sept. 1895. 
3 Askenasy, 1895, p. 11. 
* Sachs, Zert Book, edit. iv. Eng. Tr., p. 679, describes evaporation taking place 
in the cell wall, which makes good the loss by imbibition. 
° Pringsheims Jahrb. xviii. 1887, p. 1. 
§ Strasburger (Leitungsbahnen, p. 777) observed equilibrium established a good 
deal quicker. 
